Input method - current user's input method forced to change?

Well, its not definitive but so far I am tending towards

Something with sdl2 being in the AUR and have the fcitx5 build depend and the use of pamac (repo+aur).

Nope. Neither sdl2-compat nor sdl3 require fcitx5.
Even if you reverted to sdl2 from the AUR fcitx5 would still not be required except to build the package.
( Aside from not exhibiting the issue of source ‘confusion’, every competent AUR helper provides ways to automatically remove build depends after transactions. Let alone removing the AUR from the equation and using pacman alone - I doubt would have created this scenario. [Would be an interesting test if someone uses backups. :wink: ])

I dont know why you are so eager to paint this with overtures to authoritarianism.
Its just a package exchange.
( even if an undesirable one )

But yes, you can safely remove fcitx5 and related packages … as shown by the dependencies.